The long-term goals of this research are to identify and characterize the deoxyribonucleases (DNases) in human blood serum with respect to distinguishing enzymatic and physical characteristics, tissue(s) of origin, and qualitative and quantitative variations in activity that may be of value for detecting malignant diseases. Present evidence continues to support our initial assumption that the most likely origin for serum DNase I type activity is the pancreas. Blood leukocytes seem the most likely source of DNase II type activity. These hypotheses receive special emphasis in this research.